The composition of the P840-reaction center complex (RC), energy and electron transfer within the RC, as well as its topographical organization and interaction with other components in the membrane of green sulfur bacteria are presented, and compared to the FeS-type reaction centers of Photosystem I and of Heliobacteria. The core of the RC is homodimeric, since pscA is the only gene found in the genome of Chlorobium tepidum which resembles the genes psaA and -B for the heterodimeric core of Photosystem I. Functionally intact RC can be isolated from several species of green sulfur bacteria. It is generally composed of five subunits, PscA-D plus the BChl a-protein FMO. Functional cores, with PscA and PscB only, can be isolated from Prostecochloris aestuarii. The PscA-dimer binds P840, a special pair of BChl a-molecules, the primary electron acceptor A(0), which is a Chl a-derivative and FeS-center F(X). An equivalent to the electron acceptor A(1) in Photosystem I, which is tightly bound phylloquinone acting between A(0) and F(X), is not required for forward electron transfer in the RC of green sulfur bacteria. This difference is reflected by different rates of electron transfer between A(0) and F(X) in the two systems. The subunit PscB contains the two FeS-centers F(A) and F(B). STEM particle analysis suggests that the core of the RC with PscA and PscB resembles the PsaAB/PsaC-core of the P700-reaction center in Photosystem I. PscB may form a protrusion into the cytoplasmic space where reduction of ferredoxin occurs, with FMO trimers bound on both sides of this protrusion. Thus the subunit composition of the RC in vivo should be 2(FMO)(3)(PscA)(2)PscB(PscC)(2)PscD. Only 16 BChl a-, four Chl a-molecules and two carotenoids are bound to the RC-core, which is substantially less than its counterpart of Photosystem I, with 85 Chl a-molecules and 22 carotenoids. A total of 58 BChl a/RC are present in the membranes of green sulfur bacteria outside the chlorosomes, corresponding to two trimers of FMO (42 Bchl a) per RC (16 BChl a). The question whether the homodimeric RC is totally symmetric is still open. Furthermore, it is still unclear which cytochrome c is the physiological electron donor to P840(+). Also the way of NAD(+)-reduction is unknown, since a gene equivalent to ferredoxin-NADP(+) reductase is not present in the genome.

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Coxiella burnetii, the causative agent of Q fever, is an obligate intracellular gamma-proteobacterium, which replicates within large phagolysosome-like compartments formed in the host cell. The global protein profile of intracellular C. burnetii strain Nine Mile phase II was analyzed by two gel-based approaches coupled to MALDI-TOF MS. Colloidal Coomassie brilliant blue-stained 2-DE gels at the pH range 3-10 resolved over 600 protein spots and 125 spots in doubled-SDS-PAGE gels. Mass spectra obtained for each trypsin-digested protein-spot were compared to the C. burnetii genome database, and a total number of 185 different C. burnetii proteins were identified by both techniques. 2-DE in combination with MALDI-TOF MS, as a high-throughput method, allowed the identification of 172 proteins. On the other hand, the application of doubled-SDS-PAGE allowed the identification of 38 proteins, with some of them being very alkaline and membrane proteins not identified in the 2-DE approach. Most identified proteins were predicted to be involved in metabolism and biosynthesis. Several identified proteins are speculated to have a distinct and vital role in the pathogenesis and survival of C. burnetii within the harsh phagolysosomal environment.

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Light energy absorption and transfer are very important processes in photosynthesis. In green sulfur bacteria light is absorbed primarily by the chlorosomes and its energy is transferred via the Fenna–Matthews–Olson (FMO) proteins to a homodimeric reaction center (RC). Here, we report the cryogenic electron microscopic structure of the intact FMO-RC apparatus from Chlorobaculum tepidum at 2.5 Å resolution. The FMO-RC apparatus presents an asymmetric architecture and contains two FMO trimers that show different interaction patterns with the RC core. Furthermore, the two permanently bound transmembrane subunits PscC, which donate electrons to the special pair, interact only with the two large PscA subunits. This structure fills an important gap in our understanding of the transfer of energy from antenna to the electron transport chain of this RC and the transfer of electrons from reduced sulfur compounds to the special pair.